x86/signal: Change return type of restore_sigcontext() to boolean

None of the call sites cares about the return code. All they are interested
in is success or fail.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210908132525.851280949@linutronix.de